The sensible update for my vintage 2011 home machine
As it happens, I think that there is a sensible, boring answer to what I should do about my current home computer, one that bypasses all of my concerns over a potential replacement machine. It's just an answer I've been somewhat reluctant to do, partly because it's nowhere near as interesting as putting together a new machine and partly because it's going to require some awkward gyrations.
What I should really do is get a pair of 500 GB SSDs and use them to replace the current pair of system disks. A relatively small amount of space would go to an ext4 software RAID mirrored root filesystem (with all system stuff in it); the majority of it would go to a new ZFS pool where I would put my home directory and a few other filesystems where I want fast access (for example because I compile source code there). This would get me both ZFS and a speedup on a bunch of things that I do.
(At this point, some people are probably incredulous that I haven't already shifted my primary storage to SSDs.)
Following my experience with my work machine, I should probably also take a deep breath and switch my machine from 16 GB of RAM to 32 GB, because this will make ZFS happier. The RAM will not be reusable in a future machine, unlike the SSDs, and it's still surprisingly expensive, so it's tempting to skip this and see if I can make do (especially with ZFS on Linux ARC improvements).
While upgrading my entire machine is the attractive thing to do, at this point it seems very likely that the largest single practical speedup I could do is moving frequently used data to from HDs to SSDs. And I don't need a whole new machine to do that, by any means. There are limitations to what this will do for me (it won't speed up processing photos, for example), but I think it's likely to make a difference.
(Having looked at the CPU compatibility list for my Asus P8P67 LE motherboard, I don't think there's any point in trying to upgrade the CPU. In fact it basically seems like the LGA 1155 CPUs that my motherboard uses are now so old that they're basically legacy hardware that you pay extra for. Even if my perception here is wrong, it doesn't look like I can get much of a CPU upgrade.)
PS: My plan to put the root filesystem on the SSDs comes partly from necessity (it's currently on the HDs that the SSDs would replace) and partly because that's what I wound up doing on my work machine after contemplating my dilemma about this.
Sidebar: the irrational reason I've been avoiding doing things like this
For no particularly good reason, I've always found it easier to talk myself into buying a whole new machine every so often instead of progressively upgrading a machine piece by piece. As best I can figure it out, spending more money on something that's progressively becoming more and more obsolete just feels like a waste.
(As you may guess, I'm not one of the people who enjoys progressively upgrading machines piece by piece. In fact I think the only hardware changes I've done in machines to date has been replacing dead HDs.)